1. Field of the Invention
The present invention relates to a magnetic field detection apparatus that detects the angle and intensity of a magnetic field using magneto-resistive elements (hereinafter also referred to as MR (magnetoresistive) elements). The present invention also relates to an apparatus for measuring displacement that measures displacement of physical quantity such as displacement of an angle, gradient, or stroke using magneto-resistive elements (hereinafter also referred to as MR (magnetoresistive) elements). Further, the present invention also relates to a measurement apparatus using transducer elements whose resistance changes depending on the ambient physical quantity.
2. Background Art
Such a magnetic field detection apparatus or an apparatus for measuring displacement using MR elements has been known from, for example, Reference 1 (JP Patent Publication (Kokai) No. 2003-121197 A), Reference 2 (JP Patent Publication (Kokai) No. 2005-24287 A), Reference 3 (JP Patent Publication (Kokai) No. 2000-310504 A), and the like.
Among magneto-resistive elements (MR elements), there are known anisotropic magneto-resistive elements (anisotropic magnetoresistance elements; hereinafter referred to as “AMR elements”), giant magneto-resistive elements (giant magnetoresistance elements; hereinafter referred to as “GMR elements”), and the like. Hereinafter, a brief summary of the conventional techniques will be described by way of an example of a magnetic field detection apparatus using GMR elements.
A GMR element has a first magnetic layer (a pinned magnetic layer), a second magnetic layer (a free magnetic layer), and a non-magnetic layer (a spacer layer) sandwiched between the two magnetic layers. When an external magnetic field is applied to the GMR element, the magnetization direction of the pinned magnetic layer does not change and remains pinned, whereas the magnetization direction of the free magnetic layer changes in accordance with the direction of the external magnetic field.
When a voltage is applied between the both terminals of the GMR element, a current corresponding to the resistance of the element flows through the element. The magnitude of the resistance of the element changes depending on the difference Δθ between the magnetization direction θp of the pinned magnetic layer and the magnetization direction θf of the free magnetic layer (Δθ=θf−θp). Thus, if the magnetization direction θp of the pinned magnetic layer is known in advance, it is possible to detect the magnetization direction θf of the free magnetic layer, that is, the direction of an external magnetic field by measuring the resistance of the GMR element utilizing such a property.
A mechanism in which the resistance of the GMR element changes depending on Δθ=θf−θp is described below.
A magnetization direction in a thin-film magnetic film is related to the spin direction of electrons in a magnetic material. Thus, when Δθ=0, the free magnetic layer and the pinned magnetic layer have a high percentage of electrons that spin in the same direction. Conversely, when Δθ=180°, the two magnetic layers have a high percentage of electrons that spin in opposite directions.
FIGS. 3A and 3B each schematically show a cross section of a free magnetic layer 11, a spacer layer 12, and a pinned magnetic layer 13. The arrows in the free magnetic layer 11 and the pinned magnetic layer 13 schematically show the spin directions of the majority of electrons. FIG. 3A shows a case in which Δθ=0, i.e., electrons' spin directions of the free magnetic layer 11 and the pinned magnetic layer 13 are aligned. FIG. 3B shows a case in which Δθ=180°, i.e., electrons' spin directions of the free magnetic layer 11 and the pinned magnetic layer 13 are opposite. When θ=0 as shown in FIG. 3A, electrons spinning to the right, which have escaped from the pinned magnetic layer 13, are not scattered in the free magnetic layer 11 almost at all because a large number of electrons in the free magnetic layer 11 spin in the same direction. Thus, the electrons follow a trajectory as indicated by an electron trajectory 810. Meanwhile, when Δθ=180° as shown in FIG. 3B, electrons spinning to the right, which have escaped from the pinned magnetic layer 13, are scattered frequently upon entering the free magnetic layer 11 because the free magnetic layer 11 contains many electrons that are spinning in the opposite direction. Thus, the electrons follow a trajectory as indicated by an electron trajectory 810. As described above, when Δθ=180°, the probability of electron scatterings could increase, which in turn could increase the electrical resistance.
When Δθ is an intermediate value between 0 and 180°, a state between the states of FIGS. 3A and 3B results. The resistance of a GMR element is known to satisfy:R=R0+ΔR(1−cos Δθ)/2  [Formula 1]
ΔR/R equals several % to several tens of %.
As described above, a current flow through (i.e., electrical resistance of) a GMR element can be controlled with the direction of electrons' spin. Thus, it is also called a spin-valve element.
A magnetic film with a thin film thickness (a thin-film magnetic film) has an extremely large demagnetizing factor in the direction of the normal to the plane. Thus, a magnetization vector cannot rise up in the direction of the normal to the plane (the film thickness direction) and thus remains lying in the plane. Each of the free magnetic layer 11 and the pinned magnetic layer 13 of the GMR element is sufficiently thin. Thus, the magnetization vectors of the two magnetic layers lie in the direction of the plane.
A magnetic field detection apparatus has a Wheatstone bridge constructed from four GMR elements R1 to R4 as shown in FIG. 4. Herein, the magnetization direction of a pinned magnetic layer of each of R1 and R3 is set at zero (θp=0), and the magnetization direction of a pinned magnetic layer of each of R2 and R4 is set at 180° (θp=180°. The magnetization direction θf of a free magnetic layer is determined by an external magnetic field. Thus, all of the magnetization directions θf of the free magnetic layers of the four GMR elements are the same, satisfying the following relationship: Δθ2=θf−θp2=θf−θp1−π=Δθ1. Since Δθ1 is based on θp=0, it is assumed that Δθ1=0. Thus, as can be seen from Formula 1, R1 and R3 satisfy (n=1, 3):
                              R          n                =                              R            n0                    +                                                    Δ                ⁢                                                                  ⁢                R                            2                        ⁢                          (                              1                -                                  cos                  ⁢                                                                          ⁢                  θ                                            )                                                          [                  Formula          ⁢                                          ⁢          2                ]            
In addition, R2 and R4 satisfy (n=2, 4):
                              R          n                =                              R            n0                    +                                                    Δ                ⁢                                                                  ⁢                R                            2                        ⁢                          (                              1                +                                  cos                  ⁢                                                                          ⁢                  θ                                            )                                                          [                  Formula          ⁢                                          ⁢          3                ]            
The differential voltage Δv=v2−v1 between terminals 1 and 2 upon application of an excitation voltage e0 to the bridge circuit of FIG. 4 is given by:
                              Δ          ⁢                                          ⁢          v                =                              (                                                                                R                    1                                    ⁢                                      R                    3                                                  -                                                      R                    2                                    ⁢                                      R                    4                                                                                                (                                                            R                      1                                        +                                          R                      4                                                        )                                ⁢                                  (                                                            R                      2                                        +                                          R                      3                                                        )                                                      )                    ⁢                      e            0                                              [                  Formula          ⁢                                          ⁢          4                ]            
When Formula 2 and Formula 3 are substituted into Formula 4, and provided that Rn0 is equal when n=1 to 4, and also provided that R0=Rn0, the following formula is obtained.
                              Δ          ⁢                                          ⁢          v                =                                            -              Δ                        ⁢                                                  ⁢            R            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                              2              ⁢                                                          ⁢                              R                0                                      +                          Δ              ⁢                                                          ⁢              R                                                          [                  Formula          ⁢                                          ⁢          5                ]            
As described above, since the signal voltage Δv is proportional to cos θ, the direction θ of a magnetic field can be detected.
According to Formula 5, the signal voltage Δv=0 when θ=90°. That is, Δv=0 because the following proportional relationship is satisfied when θ=90°:
                                          R            1                                R            4                          =                              R            2                                R            3                                              [                  Formula          ⁢                                          ⁢          6                ]            
In practice, however, there are cases in which Δv=0 cannot be satisfied and an offset voltage remains even when θ=90°. An offset voltage is generated when the proportional relationship of Formula 6 cannot be satisfied, for example, because the electrical resistances of the four GMR elements deviate due to deviation in production quality of the four GMR elements.
Since the angle θ of a magnetic field is calculated on the assumption that the signal Δv is proportional to cos θ, measurement errors could occur if an offset voltage remains. Further, since an offset voltage changes with temperature in many cases, a change in temperature could also result in a change in measurement errors.
Countermeasures against the aforementioned problem of the generation of the offset voltage have been proposed so far. Such measures are roughly divided into the two following methods. According to the first method, a compensation resistor is built into a Wheatstone bridge, in addition to four GMR elements. This method is disclosed in, for example, JP Patent Publication (Kokai) No. 2000-310504 A (Reference 3) in which the resistance value of the compensation resistor is adjusted to satisfy the proportional relationship of Formula 6 and thus to eliminate an offset voltage. This method, however, has a problem in that since an offset voltage could change with a change in temperature, if the temperature is changed, the proportional relationship of Formula 6 cannot be satisfied, resulting in generation of an offset voltage.
According to the second method, an offset voltage is stored in advance, and the offset voltage is subtracted from a signal voltage for compensation. However, in practice, an offset voltage changes with temperature, and thus it is necessary to measure an offset voltage for each temperature within the range of temperatures to be used, in advance. Thus, a problem is posed that the production and inspection processes of modules as well as the inspection time could increase, which could result in a cost increase. Further, another problem is posed that it is also necessary to provide temperature measurement means for measuring the temperatures of the GMR elements, which could result in a complex module configuration.